What's next??? Cardboard houses are hardly new, I've seen one that sells for about 30,000 dollars that's been around for a few years. Cars are less likely due to crash standards. Tennis racquets are unlikely because the thickness of the cardboard matrix required to allow enough stiffness would impair aerodynamics of the racquet.

A racket made out of a very dense cardboard might not be a bad idea. It would essentially be like a wood racket but with little air pockets to decrease the density and allow modern headsizes. Though I doubt such a material could be made durable enough, perhaps if we mixed in a little graphite.

Mega troll but ill play
I think that it depends on where you draw the line, when does card become a diff substance. If you mix in graphite or carbon fiber is it no longer cardboard. How large do the perforations have to be before its just a thick as$ piece of paper.

The Westerm imperialist block is way behind the times, as always.
In communist Eastern-Europe we had cars made of cardboard decades ago.

(Well, actually it turns out it was made of something called Duraplast, which used recycled cotton as an ingredient, but people generally believed it was made out of corrugated cardboard. Either way, it was recycled garbage. 26hp, and famous for the heavy exhaust it generated.)